pub fn odd_fibnacci_sum(threshold: u32) -> u32 {
    // x1, x2 = 1 1
    let mut x1 = 1;
    let mut x2 = 1;

    let mut sum = 0;
    // 递推斐波那契?
    while x1 <= threshold {
        // if x1 % 2 == 1
        if x1 % 2 == 1 {
            // sum += x1
            sum += x1;
        }
        // x1, x2 = x2, x1 + x2
        let tmp = x1;
        x1 = x2;
        x2 = tmp + x2;
    }

    sum
}
